1. Field of the Invention:
The field of this invention is eye surgery. More specifically, this invention relates to surgical techniques for fistulization of the eye and to devices useful in such procedures.
2. Description of the Prior Art:
It is well known in the field of Ophthalmology that chronic inflammation within an eye can cause deterioration and gradual opacification of the cornea by slowly damaging the cells on the inside surface of the cornea. Chronic inflammation within the eye has also been shown to cause problems with the retina. It is particularly important to avoid chronic inflammation in connection with glaucoma surgery, which is performed when a patient develops difficulties from increased pressure within the eye. This increased pressure can cause damage to the optic nerve, which may be manifested by a loss of vision, or may cause changes in the visual field. Most cases of glaucoma are treatable by topical or oral medications, however in many cases the patient must undergo glaucoma surgery to relieve the pressure. A glaucoma surgical procedure typically utilizes some type of fistulization process to drain fluids out from within the eye. With some individuals, however, there is difficulty in maintaining adequate fistulization, in which case mechanical devices are often implanted in the eye to facilitate the outflow of fluid.
These mechanical devices fall into two categories: free flowing systems and pumps. Free flowing systems allow fluid to filter out of the eye, with the flow of fluid being restricted only by the size of the conduit and the development of scar tissue which forms around the filtering system. Pumping mechanisms operate to mechanically assist in the flow of fluid away from the eye.
When the eye is being drained of fluids, care must be taken to avoid hypotony, which is a condition where the internal pressure within the eye becomes lower than desired. A prolonged hypotony can lead to complications, such as shallowing or loss of the anterior chamber which can cause damage to the cornea. A hypotonic condition can also cause hemorrhaging in the posterior portion of the eye, which may result in a choroidal or retinal detachment, and also could possibly result in the loss of the eye itself. To prevent hypotony with a free flowing drainage system, sometimes sutures are placed around portions of the filtration tube to restrict the flow of fluid while scar tissue is developing. Once scarring has been formed around the filtering system, a sufficient natural restriction on drainage has been created to allow the suture to be removed and drainage to take place.
One significant disadvantage of the free flowing systems is that a large amount of inflammatory debris can sometimes pass in a retrograde fashion from the filtration system and filtering blebs back into the eye. While retrograde flow of inflammatory debris can occur at any time with such a system, this problem can be aggravated when the filtering tube is closed, as described above, to prevent hypotony. When ligatures are used to constrict the filtering tube, blood and debris fill the tube proximal to the ligature and, upon opening the filtration tube, a large amount of inflammatory debris is liberated within the eye despite the fact that initial fluid flow from the eye to the filtering system pushes the debris temporarily back up into the filtration system. This back flow of inflammatory debris into the eye is a potential cause of chronic inflammation of the eye, possibly resulting in the above described undesirable complications.
An example of a pump system for relieving intraocular pressure is shown in U.S. Pat. No. 4,554,918 to White. In order to avoid hypotony, an inherent pressure gradient is needed to be achieved across the White pump before fluid can flow through the pumping mechanism. It is suggested in U.S. Pat. No. 4,554,918 that this pressure gradient be in the range of 8-10 mm Hg. While this design characteristic of the White pump helps decrease the occurrence of hypotony immediately after implantation, sometimes the eye does not reach a sufficient pressure for fluid to exit, in which case this pump fails to function as a pressure relief mechanism. This pump also serves to restrict the retrograde flow of fluid, however debris has been found to clog the pumping mechanism, causing it to fail to function properly. U.S. Pat. No. 4,886,488 to White discloses a second pressure relief system which also operates upon the occurrence of a pressure gradient in the range of 8-10 mm Hg.